The present invention relates to a polyarylene thioether composition for molding. More concretely, the present invention relates to a composition for molding having remarkably improved anti-flash property (difficulty in occurrence of flash) and being prepared by combining a considerable amount of cross-linked polyarylene thioether which becomes gel under melting, as an anti-flash property improving agent, with the base resin of polyarylene thioether.
Polyarylene thioether (hereinafter referred to as PATE) represented by polyphenylene thioether is being used in many fields as an engineering plastic excellent in heat-resistance, non-inflammability, chemical resistance, etc. PATE is usually a crystalline polymer and, making the most of its feature, it is used particularly in the field of injection-molding, etc.
A conventional PATE as the base resin for the composition for molding is a cross-linked PATE which is prepared by curing a PATE of a relatively low molecular weight, namely, raising its apparent melt viscosity by forming cross-linked and branch-chained PATE with thermal treatment. The PATE prepared as above is excellent in processability in melt molding, such as injection molding, and has a characteristic point that it has usually an enough anti-flash property when it is injection molded.
However, on the other hand, the conventional PATE has many problems, so far. Namely, it is insufficient in mechanical strength, is brittle and is severely colored in thermal treatments. Moreover, because the time-dependent change of its viscosity (namely, the irreversible increase of its viscosity) during melt molding is severe, it is difficult to select the suitable molding conditions and to recycle its molded scraps. Accordingly, it is not economical to use.
On the other hand, the present inventors have developed a process for economically producing a substantially linear PATE of a high molecular weight by carrying out the addition of water and raising the temperature during polymerization (refer to U.S. Pat. No. 4,645,826).
Since a molecular weight of the substantially linear PATE obtained is sufficiently large, it can be applied to various melt-processings, such as injection molding, without being subjected to any further curing nor cross-linking during its polymerization reaction.
Such substantially linear and non cross-linked PATE has various characteristic points as follows:
(1) because of its molecular structure, it has a large mechanical strength; (2) as it is hardly colored in processing, it can easily be colored artificially; (3) as the time-dependent change of its melt viscosity during melt-molding is extremely small, its processing is easy; and (4) because of easiness in the recycle use of molded scraps, it is quite economical.
However, the composition having this substantially linear PATE of high molecular weight as a base resin, cannot have sufficient anti-flash properties on injection molding.
On the other hand, even in the composition having the conventional cross-linked PATE (aforementioned), which is usually favorable in its anti-flash property, as a base resin, there has been a problem that in injection molding for precision molded products, its anti-flash property is still insufficient.
The present inventors have extensively studied to improve the anti-flash property on injection molding the composition of non cross-linked PATE and also the anti-flash property on injection molding to prepare precision molded products from the conventional cross-linked PATE.
At first, the present inventors tried to improve the anti-flash property of the composition for injection molding by increasing the viscosity of its base resin, PATE. However, it was found out that viscosity of the composition increases in proportion to viscosity increase of the base resin and its processability (fluidity, etc.) for injection molding is damaged.
Then, the present inventors have studied tracing the method described in Japanese Patent Application Laid-Open (KOKAI) Nos. 59-223,753 (1984); 52-149,348 (1977); 55-127,002 (1980); 59-11,357 (1984); 53-66,565 (1978) and 53-22,363 (1978). Namely, the method comprises an addition of a PATE of a high viscosity, having a melt-flow value (measured by ASTM-D 1238: at 600.degree. F. under a load of 5 kg) of about 5 to 200 g/10 minutes, or a melt viscosity, .eta.*, (at 310.degree. C. under a shear rate of 5/second) of around 1.times.10.sup.3 to 1.times.10.sup.5 poise, to the base resin. However, it was realized that the method of adding a PATE of such viscosity, an effective improvement of the anti-flash property cannot be achieved unless a large amount of such PATE of a high viscosity is added.
Moreover, it was also found out that when a large amount of such PATE of high viscosity is added, the viscosity of the composition also increased and its processability for injection molding was remarkably damaged.
The present inventors have further studied method for effectively improving the anti-flash property of the composition without giving any substantial badinfluence on its processability for injection molding and on physical properties of the molded products. As a result, the present inventors have found that by carrying out a method which has never been considered so far, namely, a method of combining a highly cross-linked PATE (which becomes gel when melted) as an antiflash property improving agent, with the base polymer, the anti-flash property of the composition is remarkably improved without giving any substantial badinfluence on the processability and also on physical properties of the molded products. Further, side effects of the addition of this highly cross-linked PATE, for example, as a flow-orienting agent, accelerating crystallization of the composition during injection molding and, as a releasing agent, improving releasability of the molded products from molds, have also been found. Furthermore, when said cross-linked PATE is used rather in a large amount, it has also been found that not only the effects described above, but also an improvement of weld-property in injection molding is obtained.